Polymer polyols are high volume commercial products whose main use is in the production of polyurethane slab stock, high resilience, and molded foams. Polymer polyols consist of a dispersion of vinyl polymers in a continuous phase which generally comprises a polyoxyalkylene polyol. Polymer polyols have been produced by numerous methods in the past. For example, vinyl polymers have been separately synthesized and subjected to in situ particle size reduction in a polyol. So-called "redispersable graft polyols" have been prepared by first preparing vinyl polymers in small particulate form, followed by dispersing these particles in a polymer polyol. However, the most common method of producing polymer polyols has been, and remains today, the in situ polymerization of one or more vinyl monomers in a continuous polyol phase. In the present application, the term "polymer polyol" refers to polymer polyols produced by such in situ vinyl monomer polymerization.
Numerous problems have been associated with the production and use of polymer polyols. Commercially acceptable polymer polyols must have a reasonably low viscosity, i.e., below 10,000 mPa.cndot.s and preferably about 5,000 mPa.cndot.s or lower; should be stable dispersions which do not tend to settle out over time; should have a relatively narrow particle size range without the presence of large particulates; and should be white in color in order that light colored polyurethane foams may be produced. Early polymer polyols had relatively low solids content. While low solids content is not necessarily an impediment to producing a suitable polyurethane foam product, the production of polymer polyols at low solids is uneconomical. Higher solids polymer polyols may be diluted with conventional polyols for actual use.
Early on in the preparation of polymer polyols, it was believed that a "grafting" reaction took place between a portion of the vinyl monomers and the polyether polyol chain. While a grafting reaction could hypothetically take place at the allylic unsaturation sites which are present in base-catalyzed polyoxypropylene polyols, the allyl group is particularly unreactive relative to other types of ethylenic unsaturation. Thus, many researchers believed that grafting reactions, if they did in fact take place, occurred through abstraction of hydrogen atoms from the alkylene groups of the polyether chain rather than by reaction of the unsaturated allyl group. Regardless of the mechanism by which the polymerization takes place, and regardless of whether grafting in fact occurs, early polymer polyols suffered from relatively low solids content, and were often highly colored as well, ranging from tan to brown to reddish-orange in color. Furthermore, attempts to increase the solids content often led to "seedy" polyols having numerous particles of large particle size which could not be readily filtered; produced very viscous products; or resulted in gelling of the reactor with rather disastrous consequences, necessitating expensive and time consuming reactor clean-up.
It was subsequently discovered that through the purposeful addition of more reactive unsaturation sites into the polyether molecule, polymer polyols of higher solids content and greatly reduced color and viscosity could be obtained. Moreover, it was also discovered that only a relatively small fraction of the total number of polyol molecules need contain unsaturated sites. Apparently, the reaction of the vinyl monomers with the more reactive sites resulted in the production of molecules which acted as stabilizers for the dispersion, preventing the agglomerization of small vinyl polymer particles into large particles, and also preventing coagulation and gelling of the reactor. The stabilizers produced by these reactions are termed "steric stabilizers", as they are believed to function by sterically hindering the agglomerization and/or coagulation of vinyl polymer particles into larger particles.
Steric stabilization may be entropic and/or enthalpic. One can envision an associating population of vinyl polymer particles having relatively long polyether polyol chains extending into space around the particles. The enthalpic changes which occur during particle association are mainly the result of electronic interactions which the various portions of the particles have with the continuous polyol phase and with other polymer particles. The entropic changes are reflective of the decreased degrees of freedom which the extending polyol chains have as the particles agglomerate. The anti-agglomerative effect achieved by entropic stabilization is derived from the decrease in entropy of the polyol portion of the stabilizer molecule which occurs as particles agglomerate. In other words, the number of degrees of freedom that the polyol portion of the stabilizer can assume in space is decreased as two particles approach. Accordingly, the entropy of the suspension is maximized in the non-agglomerated state.
Several different types of polymer polyol stabilizers have evolved. The earliest stabilizers, sometimes termed "macromonomers" or "macromers", were prepared by the reaction of a polyoxyalkylene polyol with maleic acid followed by isomerization of the maleate cis-double bond to the more reactive fumarate trans-double bond. The products of this reaction were polyetheresters containing a fumarate half-ester moiety. The polyoxyalkylene polyol half-ester could be used as such for a stabilizer precursor, or could be further reacted with alkylene oxide, or esterified with a glycol, to remove the remaining carboxylic acid functionality and replace it with primary or secondary hydroxyl functionality. These "macromonomers" are not stabilizers per se, but form stabilizers during vinyl polymerization. Thus, they may appropriately be called "stabilizer precursors." Such stabilizer precursors have been widely used, and continue to be used to the present day. However, such stabilizers are relatively expensive to prepare due to the relatively long process time which often requires approximately eight hours or more.
Rather than employ maleic anhydride to induce fumarate unsaturation into a stabilizer precursor, molecules containing a hydroxyl-reactive isocyanate group together with a site of reactive ethylenic unsaturation may be used to prepare stabilizer precursors. An example is the use of isocyanatoethylmethacrylate and similar compounds which may be prepared by reacting a hydroxyl functional acrylate such as 2-hydroxyethylacrylate with an excess of diisocyanate. Stabilizer precursors such as these, having very reactive acrylic unsaturation, have been also widely used for polymer polyol production. Unfortunately, compounds such as isocyanatoethylmethacrylate often exhibit storage stability problems, and often must be prepared just prior to use, thus reducing the flexibility of such processes on an industrial scale. A further, more recent example of a functionalizing reactant which may be used to induce unsaturation is "TMI", 1-(t-butyl-isocyanato)-3-isopropenylbenzene.
An alternative approach to the use of "stabilizer precursors" or "macromers" is the use of so-called "preformed stabilizers". As with stabilizer precursors, the manufacture of preformed stabilizers begins by adding induced reactive unsaturation onto a polyoxyalkylene polyol molecule. However, rather than utilize this stabilizer precursor directly in the preparation of polymer polyols by in situ polymerization of vinyl monomers, a limited polymerization of vinyl monomers is first conducted in the presence of the stabilizer precursor. In one approach using preformed stabilizers, very limited vinyl polymerization in the presence of the stabilizer precursor results in a low molecular weight polyoxyalkylene/polyvinyl polymer which remains soluble in the polyol. This process is illustrated by Published International Application WO 87/03886, but is not known to have led to commercial products. It is believed that the vinyl polymer polyol viscosities produced using soluble preformed stabilizers are too high for commercial acceptance.
In a second preformed stabilizer process, the initial vinyl polymerization is continued until a vinyl polymer particle dispersion having a relatively low solids content, i.e., from 3-15 weight percent, is obtained. This vinyl polymerization may be conducted with a relatively high amount of free radical polymerization initiator and chain transfer agent, which encourage the formation of large numbers of relatively small vinyl particulates. The mean particle size may often be one micrometer or less, for example. These preformed stabilizers are translucent or opaque, indicating that a dispersion rather than a solution of preformed stabilizer has been obtained. These preformed stabilizers may also contain some portion of soluble species.
By whichever method the preformed stabilizer is produced, polymer polyols are prepared by further polymerization with vinyl monomers which may be the same or different than those initially used, generally in the presence of a "carrier polyol" or "base polyol". The carrier polyol generally does not contain any induced unsaturation, and comprises the continuous phase. As with the stabilizer precursor process, the initial, induced unsaturation-containing molecule may be prepared with fumarate-type unsaturation, or through reaction with isocyanate group-containing unsaturated compounds such as isocyanatoethylmethacrylate. Other reactive unsaturated compounds such as TMI may be used as well. The preformed stabilizer process has certain advantages over the stabilizer precursor process, however, in that once prepared, the preformed stabilizer is s table and can be stored for extended periods of time prior to use in preparing the final polymer polyol. By whichever method polymer polyols are produced, these polymer polyols may obtain solids contents as high as 60% or more while achieving relatively low viscosity and being either white or slightly off-white in color. The products achieve acceptable filterability as well, indicating a lack of large size particles.
Stabilizer precursors and preformed stabilizers are both relatively expensive polymer polyol starting materials. When maleic anhydride is used to prepare a stabilizer precursor or a preformed stabilizer, a large part of the expense of the stabilizer is connected with extended processing time. In the case of induced unsaturation derived from isocyanatoethylmethyacrylate, the expense is due more to the expensive nature of the isocyanatoethylmethacrylate monomer rather than the processing time. In either case, however, it is clear that minimizing the amount of steric stabilizer necessary to prepare the final polymer polyol is highly desirable.
It has been discovered, as illustrated by U.S. Pat. Nos. 4,954,561 and 5,494,957, that the degree of steric stabilization can be markedly increased if the polyether polyol portion of the stabilizer molecule is increased in size through coupling of relatively high molecular weight polyols into yet higher molecular weight coupled products. Coupling is achieved in U.S. Pat. No. 4,954,561 by coupling polyols through use of oxalic acid, forming an oxalate diester, while in U.S. Pat. No. 5,494,957, coupling of stabilizer precursors is obtained through reaction with a diisocyanate. Through such coupling, stabilizer efficiency is improved, allowing for use of smaller portions of stabilizer. However, this increase in efficiency is offset, at least in part, by an increase in raw material cost and processing time due to the separate coupling reaction.
It would be desirable to provide stabilizer precursors and preformed stabilizers which can be used in lower proportions in the production of polymer polyols, and/or which allow the production of polymer polyols having improved properties such as filterability, particle size, lower viscosity, and the like. Such stabilizers should be capable of economic preparation without extended processing time, and in particular, should provide efficient stabilization without the need for coupling.